When a fuel cell is in operation, the cathode has a relatively high potential of 0.6 V or more with respect to the normal hydrogen electrode (hereinafter referred to as NHE). When the operation is stopped (open circuit), the cathode has a high potential of 0.8 V or more. At such high potential, it is believed that the surface of a catalyst metal such as platinum in the cathode is oxidized and its active sites are decreased, thereby resulting in degradation of power generation performance.
To address this problem, Japanese Laid-Open Patent Publication No. Sho 63-26961 (hereinafter “JP-A 63-26961”) proposes a method of operating a fuel cell system, in which an oxidant supply line is provided with a flow rate adjust valve and the supply of an oxidant is temporarily suspended with a fuel being supplied. When the oxidant supply is suspended, the oxidant concentration in the cathode lowers and the cathode potential promptly lowers, so that the surface of the platinum catalyst in the cathode is reduced and clear active sites are exposed. This is probably the reason of the voltage rise as shown in FIG. 2 of JP-A 63-26961, although there is no such statement in JP-A 63-26961.
The power generation of fuel cells is controlled by the constant current method in which the current is kept constant or the constant voltage method in which the voltage is controlled, and either one of the two methods is selected depending on the system configuration. In JP-A 63-26961, it appears that the power generation in a steady state is controlled by the constant current method.
According to the constant current method, when the oxidant supply is suspended, the concentration overvoltage rises due to the decrease in the oxidant concentration in the cathode, but the current is maintained constant. Hence, the cathode potential promptly lowers and the surface of the platinum catalyst in the cathode is reduced and reactivated. Even in the case of a fuel cell stack composed of a plurality of cells connected in series, the cathode potential lowers in all the cells, although there is a time lag among the cells.
However, if the total voltage of such a fuel cell stack is controlled by the constant voltage method, the following problem occurs upon the suspension of the oxidant supply. That is, in some cells, the cathode potential lowers and the voltage lowers, but in some other cells, the voltage rises so as to make up for the voltage drop of other cells.
This phenomenon probably results from the difference in cathode potential between cells in which a large amount of oxygen remains in the cathode and cells in which a small amount remains. Therefore, the cathode catalyst metal such as platinum is reactivated due to reduction in some cells, but not in other cells.